Tunicates and not cephalochordates are the closest living relatives of vertebrates

Show some backboneA phylogenetic comparison of the protein sequences of 146 genes from 14 deuterostome species has come up with a result that could alter current thinking on the origin of the vertebrates. Based on overall morphology and on complexity, it was thought that the cephalochordates (marine organisms known as lancelets, or in old textbooks as amphioxus) were the vertebrates' closest living relatives. Closer than the tunicates (appendicularians, salps, and sea squirts), that were regarded as the earliest chordate lineage. But the new data suggest that tunicates, and not cephalochordates, are the closest living relatives of vertebrates. As well as the implications for vertebrate origins, this has a bearing on developmental studies in which tunicates and cephalochordates are used as model animals.